Within the realm of internal combustion engines (ICEs) used for vehicular propulsion, the spark ignition (SI) engine and the compression ignition (CI) engine are the ones most commonly in use. In SI engines, a mixture of air and fuel (typically gasoline) is introduced into a cylindrical combustion chamber for ignition via spark plug. As the resulting flame front propagates through the combustion chamber from the initial ignition point, the temperature continues to rise, which in turn leads to high peak combustion temperatures. Common emissions from the SI process include (among others) carbon monoxide (CO), unburned hydrocarbons and nitrogen oxides (NOx). Modern SI engine vehicles typically employ a three-way catalytic converter for reduction of each of these combustion byproducts. In order to ensure that the catalysts operate at peak performance with regard to NOx reduction, such engines are operated at or near stoichiometric fuel-to-air ratios as a way to avoid the presence of catalyst-impairing excess oxygen (O2) in the exhaust gas.
In CI engines, fuel (typically diesel fuel) is introduced into the combustion chamber where the air is already present in a highly compressed form. The elevated temperature within the chamber that accompanies the increased pressure causes the fuel to auto-ignite. The combustion process follows via mixing of fuel/air through diffusion. Unlike typical SI engines, the direct fuel injection of CI engines eliminates throttle losses and associated fuel pumping losses. The high compression ratio and inherent lean burn of CI engine with less pumping loss result in higher thermodynamic efficiency than that of SI engine. However, the excess O2 associated with such lean fuel-to-air ratios makes the use of the SI engine's three way catalyst impractical for NOx reduction, as such catalysts only operate effectively over a very narrow range (typically between about 0.5% and 1.0%) of O2 levels. To overcome this limitation, complicated and expensive fuel injection and after-treatment system are included. Examples of known after-treatment approaches include a lean NOx trap, selective catalytic reduction (SCR) and Exhaust Gas Recirculation (EGR). While useful for their intended purpose, excessive reliance on these approaches can be counterproductive, as lean NOx traps and SCR require periodic regeneration, while the effectiveness of EGR is limited at high engine power levels.
Another engine type, known as the homogeneous charge compression ignition (HCCI) engine, attempts to combine features of both diesel-based CI engines and gasoline-based SI engines. In an HCCI engine, the fuel and air is mixed prior to introduction to the combustion chamber in a manner generally similar to traditional SI engines. Like traditional CI engines, combustion is achieved through auto-ignition of the mixture at certain temperature and pressure conditions within the combustion chamber. Under ideal conditions, an HCCI engine achieves a diffuse, flameless combustion along with lower peak pressure and temperature; such combustion is effective in keeping NOx production low. In reality, however, it has proven extremely difficult to establish precise control over attaining the correct combination of temperature, pressure and composition of the fuel-air mixture over a wide range of engine operating conditions, particularly as it relates to high power output conditions.
Still another engine type, known as the gasoline compression ignition (GCI) engine (also referred to as the partially premixed compression ignition (PPCI) engine), has received attention as an attractive alternative to traditional diesel CI or gasoline SI engines. In a GCI engine, the fuel is injected in a staged manner late in the compression phase of the engine's four-stroke cycle operation. In this way, the fuel charge may be thought of as having both locally stoichiometric and globally stratified properties. Significantly, gasoline in general (and low-octane gasoline in particular) has a higher volatility and longer ignition delay relative to diesel fuel. By introducing such a fuel relatively late in the compression stroke and taking advantage of the fuel's ignition delay (which helps to promote additional fuel-air mixing), combustion does not commence before the end of the injection. To achieve a desirable degree of stratification, multiple injections may be used. By operating under low temperature combustion (LTC) conditions, GCI engines can have significantly reduced NOx production and soot emissions while achieving diesel-like thermal efficiencies. Moreover, refining low-octane gasoline is easier (and less expensive) than processing conventional gasoline and diesel fuels, which in turn reduces the entire well-to-tank emissions of other undesirable substances, such as CO2.
One shortcoming with a GCI engine is that the NOx emission that is low during normal engine operating conditions tends to increase under heavy loads. As such, automobile manufacturers have implemented some of the various techniques mentioned above (i.e., SCR, EGR, etc.) as a way to ensure NOx compliance under all engine operating conditions. Such corrective measures are expensive and, as mentioned above, can limit engine performance. For example, excessive EGR can lead to combustion instability and associated misfiring, as well as contribute to valve clogging from the carbon deposits that are contained in the recirculated exhaust gas. The excessive reliance on EGR also makes it difficult to control GCI engine transient responses, as the reduction of the intake air charge density may be incompatible with maintaining a continuous combustion flame front within the combustion chamber.